Melt overflow casting is a type of rapid solidification in which a metal product, such as a thin, short fiber or a wide, continuous strip, is formed by casting molten metal against the outer, peripheral surface of a rotating, cylindrical casting wheel. The molten metal solidifies on the casting wheel, is carried out of the molten pool and projected off of the casting wheel onto a surface such as a conveyor. Fibers cast onto the conveyor become entangled, forming a continuous mat. Melt extraction rapid solidification is taught in U.S. Pat. Nos. 3,871,439 and 3,838,185, both to Maringer, et al. and melt overflow rapid solidification is taught in U.S. Pat. No. 4,930,565 and U.S. Pat. No. Re. 33,327 to Hackman, et al.
The dimensions of the fiber mat or strip formed by a melt overflow process are controlled by a number of variables. Examples of these variables are the height of the liquid metal interfacing against the casting wheel, the velocity of the casting wheel and, especially for fibers, a large number of which are formed across the width of the casting wheel, the net width of fibers being produced. If a metal piece solidifies in the molten pool near the casting wheel and blocks fibers from being produced in that region of the casting wheel, then the net width is reduced and fewer fibers will be projected from the casting wheel. The total number of fibers projected from the casting wheel per unit of time will decrease. The fibers are commonly projected onto a moving conveyor to form a mat, and fewer fibers being projected onto the conveyor per unit of time will result in a mat of lower density. Usually, the interfering piece of metal which has solidified near the casting wheel and blocks the fiber from being produced is eliminated by scraping, after which the mat will return to its normal density. However, the overall result of this variation in the number of fibers being projected per unit of time is a fibrous mat having a discontinuous linear density.
Most melt overflow casting machines use some method of maintaining the height of the top surface of the molten metal pool. For example, a ceramic body may be submersed down into the pool at a specified rate as in U.S. Pat. No. 4,977,951 to Hackman. The rate at which the ceramic body displaces metal is equal to the rate at which metal is removed from the pool in order to maintain a constant pool height with respect to the casting wheel. However, if a portion of the width of the casting wheel that would normally produce fibers is blocked, then the dimensional control of the product is reduced, the surface level of the pool will rise and the remaining regions that can produce fibers will produce fibers of greater thickness or strip than if the entire width of the casting wheel were able to produce fibers. This is due to the fact that in general, the rate of descent of the ceramic plunger is set at a specified value of overflow or displacement rate in order to achieve a specified mass of output of metal per unit of time. If the number of fibers being projected off of the casting wheel decreases, but the output mass per unit of time from the molten pool remains constant, then the mass of each fiber that is produced will be greater. This results in a mat having inconsistently sized fibers which detracts from the desired homogeneity of the mat.
In the formation of a continuous, wide strip, the strip is projected onto a conveyor and has a constant width. The linear volume of the strip is primarily a function of the thickness of the strip and is desirably kept substantially constant as in the case of the fiber mat. The width of the strip may fluctuate slightly during casting, but since width only varies a small amount in relation to the total width, the most important parameter with respect to linear volume in strip casting is the thickness. If the thickness varies along the length of the strip, then the linear volume fluctuates significantly, due to the relation of a fluctuation in thickness to the total thickness. Since the thickness of the strip is much smaller than the width of the strip, a small fluctuation in the thickness represents a greater proportion of the desired thickness than the same amount of fluctuation in the width. Therefore, thickness fluctuations cause significant non-uniformities in the strip produced.
The velocity of the casting wheel and the height of the molten metal pool against the casting wheel have a large effect on the thickness of the strip just as they do in fiber casting. Generally, a more rapidly rotating casting wheel provides thinner strip and fiber.
There are a great many variables involved in forming substantially constant linear volume strip and fiber mat. Therefore there is a need for a means and method for controlling the linear volume of the two types of cast metal products in response to variations in the manufacturing parameters in order to produce uniform products.